1. Field of the Invention
The present invention concerns an arrangement for controlling individual antennas of an antenna arrangement in a magnetic resonance apparatus.
2. Description of the Prior Art
Interaction between the tissue of a patient to be examined and the basic magnetic field used for magnetic resonance excitation occur in magnetic resonance apparatuses with basic magnetic field strengths that are greater than three Tesla. Such interaction leads to shadows in the tissue images acquired from the examination. These shadows make the clinical evaluation of the tissue images more difficult.
In such magnetic resonance apparatuses it is known to use multi-channel transmission systems in order to achieve a homogenization of the B1 field and in order to minimize the aforementioned shadows in the imaging.
A multi-channel transmission system has a number of transmission units that are independent of one another, each with a transmission power amplifier. An antenna arrangement with a number of antenna elements for radiation of radio-frequency transmission signals is used for the magnetic resonance examination. A transmission unit is permanently associated with each individual antenna element of the antenna arrangement. The number of the required transmission units independent of one another thus increase with increasing number of antenna elements.
Power reflected by an antenna element acts on the individual transmission power amplifier of the transmission unit associated with that antenna element.
The individual antenna elements additionally couple with one another. A portion of the radiated transmission power each considered transmission unit is “overcoupled” into further transmission units due to the coupling. Reflected and overcoupled power affect the outputs of the respective transmission power amplifiers.
To minimize the influence of the reflected transmission power on the transmission power amplifier, either one-way conductors or circulators could be connected between the transmission power amplifiers and the respective antenna elements. Such components, however, are disadvantageously lossy in the transmission direction, or require a static magnetic field for operation, which can be achieved only with difficulty in the operation of a magnetic resonance apparatuses.
FIG. 5 shows an arrangement for controlling individual antennas A1 through A8 of an antenna arrangement BKA (designed as a “birdcage”) according to the prior art.
In total N=8 parallel transmission branches SZ51 through SZ58 are provided as transmission units corresponding to the eight individual antennas A1 through A8.
Each individual transmission branch SZ51 through SZ58 respectively comprises a modulator MOD, a transmission power amplifier PA and a transmission-reception unit Tx/Rx.
A transmission signal is modulated by the modulator MOD dependent on the transmission branch SZ51 through SZ58, amplified with the aid of the transmission power amplifier PA and arrives at an antenna element A1 through A8 (associated with the transmission branch SZ51 through SZ58) of the antenna arrangement BKA via the transmission-reception device Tx/Rx.
The transmission-reception devices Tx/Rx could thereby be designed as time-controlled cross-over switches, for example.
Received signals arrive at a common reception branch EZ51 (which has a receiver REC and an analog-digital converter ADC) from the individual antennas A1 through A8 via the correspondingly switched eight transmission-reception devices Tx/Rx. The received signals are thus digitally converted and relayed for further processing.